Semiconducting organic polymers

ABSTRACT

There is disclosed a semiconducting organic polymer polymerized from a 1-substituted, 3-substituted or 1,3-substituted indole monomer and a gas sensor containing this polymer as the active gas sensing element.

This invention relates to semiconducting organic polymers which may beused in gas sensors.

It is known that certain electrochemically prepared semiconductingpolymers such as polypyrrole may be employed in sensors in order todetect gases, vapours and odours. Such a sensor may comprise a pair ofelectrodes mounted on a substrate, with a layer of the semiconductingorganic polymer deposited on and between the electrodes in order toproduce an electrical connection between the electrodes. Thesemiconducting organic polymer may be sensitive to the presence of a gasor, more likely, to a range of gases, to the extent that adsorption ofthe gas onto the polymer surface affects the physical and electricalproperties of the polymer. Hence the presence of gas may be detected bymonitoring, for example, the change in DC resistance of the sensor onexposure to the gas. For instance, Miasik et al (Miasik, J J, Hooper, Aand Tofield, B C) J C S Faraday Trans. 1, 1986, 82, 1117-26 demonstrateda polypyrrole gas sensor displaying a DC resistance which was sensitiveto the presence of nitrous oxide and hydrogen sulphide. GB-2, 203, 553-Bdiscloses an improved method of detection wherein various AC impedancecharacteristics are measured at different AC frequencies.

A given semiconducting organic polymer will typically be sensitive to arange of compounds. Clearly this lack of selectivity is a major problemif one wishes to develop a sensor which is specific to a particular gas.Conversely, a sensor which employs a given semiconducting organicpolymer will not be sufficiently sensitive to such a broad range ofgases that it may be considered a general purpose device.

A solution to these problems is a device which employs a plurality ofsensors, wherein each sensor incorporates a different polymer and eachpolymer possesses differing gas response profiles. Thus a suite ofpolymers may be selected which possess broadly overlapping responses,but which are individually chemically tailored to enhance differences inresponse to certain molecules or classes of molecules. Often thevariation of a substituent group on the monomer unit is sufficient toenable such "fine tuning" of response. A multi-sensor device detectsgases and odours as a characteristic pattern of individual responsesacross the array of sensors.

The present invention relates to a class of semiconducting organicpolymers based on 1-substituted, 3-substituted and 1,3 substitutedindole monomer units which display high sensitivity towards a number ofimportant species.

According to a first aspect of the invention there is provided asemiconducting organic polymer polymerised from a 3-substituted or1,3-substituted indole monomer.

According to a second aspect of the invention there is provided asemiconducting organic polymer polymerised from a 1-substituted,3-substituted or 1,3-substituted indole monomer for use in a gas sensor.

A substituent at the 3 position may be an alkyl or aromatic acetylgroup.

A substituent at the 1 position may be an alkyl group.

Alternatively, a substituent at the 1 position may contain an aromaticgroup. Said substituent may be tosyl or benzyl.

The substituted indole monomer may be polymerised electrochemically froma solution containing said monomer and a counter-ion. The counter-ionmay be BF₄ ⁻, PF₆ ⁻, ClO₄ ⁻, C₈ H₁₇ SO₃ ⁻,[Fe(CN)₆ ]³⁻ or CH₃ C₆ H₄ SO₃⁻.

The semiconducting organic polymer may be polymerised from a monomerselected from the group comprising:

1-octylindole; 1-benzylindole; 1-tosylindole; 1-tosylindole;3-hexanoxyl-1-tosylindole; 3-hexanoylindole; 3-hexylindole;3-dodecanoyl-1-tosylindole; 3-dodecanoylindole and 3-dodecylindole.

According to a third aspect of the invention there is provided a gassensor comprising:

a pair of electrodes;

one or more semiconducting organic polymers, of which at least one ispolymerised according to the first aspect of the invention, depositedbetween the pair of electrodes in such manner as to effect asemiconducting electrical connection between said electrodes;

means for applying electric signal across the electrodes; and

detection means for detecting a chosen electrical property in thepresence of a gas.

Semiconducting organic polymers in accordance with the invention willnow be described with reference to the accompanying drawings in which:

FIG. 1 shows a plan view of a gas sensor;

FIG. 2 shows the view from below the gas sensor;

FIG. 3 shows the electrochemical polymerisation process;

FIG. 4 shows reaction schemes for the synthesis of substituted indoles;

FIGS. 5A-5D shows the response of some polymers to acetic acid;

FIGS. 6A-6D shows the response of some polymers to propanoic acid;

FIGS. 7A-7D shows the response of some polymers to butyric acid;

FIGS. 8A-8D shows the response of some polymers to valeric acid;

FIGS. 9A-9D shows the response of some polymers to isovaleric acid;

FIGS. 10A-10D shows the response of some polymers to phenol;

FIGS. 11A-11D shows the response of some polymers to p-cresol; and

FIGS. 12A-12D shows the response of some polymers to 4-ethylphenol.

Semiconducting organic polymers may be produced by the polymerisation of1-substituted, 3-substituted and 1,3-substituted indole monomers. Suchpolymers are particularly useful in the manufacture of gas sensingdevices of the type described hereinbefore, because as a class theyexhibit high sensitivity towards important species such as thiols andphenols. These sensitivities can be one to two orders of magnitudegreater than those displayed by the polyprrole based sensors commonlyemployed in the art. By judicious variation of substituent groups thepolymer can be further "fine tuned" to respond more selectively withrespect to the functional groups present in the detected molecule, or tomolecular size and shape.

The substituent at the 3 position may be an alkyl or acetyl group. Inparticular, large (possessing 6 or more carbon atoms), bulky substituentgroups confer greater selectivity on the resulting polymer, since stericconsiderations dictate that molecules must be of a certain size and/orshape in order to adsorb onto the surface of the polymer.

The substituent at the 1 position may also be alkyl, and a similarrationale indicates that this alkyl group is preferably large.

Alternatively, substituents at the 1 position may contain an aromaticgroup. For example, benzyl or tosyl substituents may be employed.

It will be appreciated that the foregoing discussion is not intended tobe limiting in scope, and that other types of substituent groups, forexample, a 3-substitutent containing an aromatic group, may beadvantageously employed in accordance with the present invention.Similarly, it is not intended to limit the scope of the invention to gassensor devices; polymers of the present invention may have applicationin any field known to employ semiconducting organic polymers.

FIG. 1 shows one embodiment of a gas sensor based on a modified 40 pinsilicon chip carrier 10 (Hybritek 40 L CC), wherein the gold pins 12 ofthe carrier are patterned onto a ceramic substrate 14. Adjacent pins14(a) and 16a act as electrodes, and a layer of semiconducting organicpolymer according to the present invention 18 is deposited so that thereis a semiconducting electrical connection between the electrodes. Theelectrodes are connected to plugs 14b and 16b, located on the undersideof the chip carrier. Leads are attached to the plugs 14b and 16b inorder to apply a DC potential across the electrodes and the resistanceof this electrical circuit is measured by known means (see, for example,B A Gregory; "An Introduction to Electrical Instrumentation andMeasurement Systems", 1982, MacMillen.) When the sensor is exposed to agas to which the polymer is sensitive, the presence of the gas isdetected by a variation in the DC resistance of the circuit.

In order to produce the polymer in its conducting form anelectrochemical polymerisation process is employed. The polymerisationmay be carried out by electrolytic oxidation of the monomer in anelectrochemical cell. FIG. 3 shows the electrolytic oxidation of3-methyl indole in an electrochemical cell 30. The chip carrier 10 isconnected, at 14b and 16b, to the anode 32 of the cell. The cell alsocomprises a cathode 34, a standard calomel reference electrode 36 and isflushed with nitrogen through ports 38. The anode is at 2 V with respectto the reference electrode 36. The electrolyte comprises 0.1M of themonomer and 0.1M tetraethylammonium p-toluenesulphonate in a 99%acetonitrile/1% water medium.

In solution the tetraethylammonium p-toluenesulphonate yields thetosylate anion, which is incorporated into the polymer film duringpolymerisation as a counter-ion to ensure overall electrical neutralityin the polymer. Other counter-ions may be employed including, forexample, BF₄ ⁻, PF₆ ⁻, ClO₄ ⁻,C₈ H₁₇ SO₃ ⁻ or [Fe(CN)_(6]) ³⁻. Variationof the counter-ion is another means by which the responsecharacteristics of a polymer may be moderated.

FIG. 4 shows reaction schemes for the synthesis of nine substitutedindole monomers. The reactions are described in more detail below. Allnmr data is reported in ppm and recorded at 300 MHz.

Preparation of 1-octylindole (3a)

A mixture of indole (1) (0.03 mole, 3.51 g), potassium hydroxide (0.03mole, 1.97 g) powdered in a mortar, and 18-crown-6 ether (0.001 mole,250 mg) in benzene (20 cm³) is heated under reflux with vigorousstirring for two hours. A solution of octylbromide (2a), (0.04 mole,7.72 g) in benzene (10 cm³) is added and reflux is maintained for anadditional four hours. The reaction is monitored by TLC and the reactionmixture is filtered on Celite. Evaporation of the solvent yields anorange liquid which is purified by distillation in vacuo. The colourlessliquid distilled at 154-156° C. and 1.5 mmHg pressure is shown by ¹ Hnmr to be pure 1-octylindole (3a) (4.98 g, 0.0217 mole, 72% yield).

[Found; C,83.5; H,10.2; N,6.4%: Calc. for C₁₆ H₂₃ N; C,83.8, H, 10.04;N, 6.1%]

¹ H nmr (in CDCl₃): 1.0 (t,3H); 1.37 (m, 10H); 1.88(m,2H); 4.13(t,2H);6.59 (d,1H); 7.12(d,1H); 7.2(t,1H); 7.3 (t,1H); 7.42(d,1H); 7.75 (d, 1H)

Preparation of 1-benzylindole (3b)

A mixture of indole (1) (0.03 mole, 3.51 g), potassium hydroxide (0.03mole, 1.97 g)) powdered in a mortar and 18-crown-6 ether (0.001 mole,250 mg) in benzene (20 cm³) is heated under reflux with vigorousstirring for two hours. A solution of benzyl chloride (26) (0.04 mole,5.06 g) in benzene (10 cm³) is added to the above mixture and reflux ismaintained for an additional four hours. The reaction is monitored byTLC. The reaction mixture is filtered on Celite. Evaporation of thesolvent gives an orange liquid which is purified by dry columnchromatography eluted using a mixture of dichloromethane in 50%petroleum ether. Evaporation of the solvent gives a colourless thickliquid which is solidified in the presence of diethyl ether to give awhite solid of 1-benzylindole (3b) (1.2 g, 0.007 mole, 25%), m.p. 48-51°C., shown to be pure by ¹ H nmr.

[Found; C, 86.6; H, 6.4; N, 6.8%: Calc. for C₁₅ H₁₃ N; C,86.9; H,6.28;N, 6.76%]

¹ H nmr (in CDCl₃ /TMS): 5.2 (s, 2H); 5.41 (d,1H); 6.93-7.2 (complex,9Ar-H), 7.55 (d.d, 1H)

Preparation of 1-tosylindole (4)

A mixture of indole (1) (0.157 mole, 18.4 g), dichloromethane (100 cm³),tetrabutylammonium hydrogen sulphate (0.016 mole, 5.3 g) and sodiumhydroxide solution (128 cm³, 50% aqueous) is made in 3-necked flask (500cm³). The stirred mixture is cooled in water and p-toluenesulfonylchloride (45.7 g, 0.24 mole) in dichloromethane (20 cm³) is added fairlyrapidly so that the mixture refluxes gently. When the addition iscomplete the mixture is stirred at room temperature for a further 20hours.

The inorganic precipitate is collected on Celite by suction filtration.The organic layer is separated using a separating funnel, washed fivetimes with water, and dried over anhydrous magnesium sulphate (MgSO₄).The aqueous layer is diluted with an equal volume of water and thenextracted with dichloromethane. The dichloromethane extracts are washedwith water, combined with the organic material, then dried overanhydrous magnesium sulphate. Evaporation of the solvent gives a thickoil, which is crystallised from dichloromethane and hexane to givepinkish crystals. Recrystallisation from dichloromethane and hexaneaffords cream white crystals of 1-tosylindole (4) (32.3 g, 0.119 mole,76%), m.p. 74-76° C.

[Found; C, 66.2; H,4.5; N, 5.0; S,12.0: Calc. for C₁₅ H₁₃ NSO₂ ; C,66.4;H,4.7; N,5.2; S,11.8]

¹ H nmr (in CDCl₃ /TMS): 2.21 (S, 3H), 6.55 (d, 1H), 7.05-7.25 (m, 4H,Ar), 7.45 (d.d, 1H), 7.7 (d.d, 1H), 7.9 (d,1H)

Preparation of 3-hexanoyl-1-tosylindole (6a)

Aluminium trichloride (6.01 g, 45.2 mmol), is suspended indichloromethane (80 cm³), under nitrogen. A solution of hexanoylchloride (5a) (6.62 g, 49.2 mmol) in dichloromethane (20 cm³) is addedslowly to the stirred mixture at ambient temperature. After the additionis complete, the mixture is stirred for a further 10 minutes. Thereaction mixture is cooled to 5° C. 1-tosylindole (4) (45.2 mmol, 12.24g) in dichloromethane (20 cm³) is added to the mixture while thetemperature is maintained at 5° C. After two hours stirring at roomtemperature the reaction mixture is treated with ice water, extractedwith dichloromethane and dried over magnesium sulphate. Evaporation ofthe solvent gives a thick oil, which crystallises in the presence ofmethanol and dichloromethane to yield colourless crystals of3-hexanoyl-1-tosylindole (6a) (13.07 g, 35.42 mmol, 78%), m.p. 91-93° C.

[Found: C, 68.1; H, 6.1; N,3.8; S, 8.5: Calc. for C₂₁ H₂₃ NSO₃ ; C,68.3; H, 6.2; N,3.8; S,8.6]

¹ H nmr (in CDCl₃ /TMS): 0.93 (t,3H); 1.38(m,4H); 1.76 (m,2H); 2.36(s,3H,Me in Ts); 2.86 (t,2H); 7.25 (d,2H Ar in Ts); 7.35 (M, 2H inindole); 7.81 (d, 2H Ar in Ts); 7.92 (dm, 1H), 8.21 (s, 1H in indole),8.35 (dm, 1H in indole)

Preparation of 3-hexanoylindole (7a)

3-hexanoyl-1-tosylindole (6a) (3.96 g, 10 mmol) is dissolved in dioxane(60 cm³). Sodium hydroxide (5M, 60 cm³) is added to the solution whichis stirred under reflux for 24 hours. The organic phase is separatedfrom the aqueous layer by extraction with diethyl ether. The combinedextracts are washed with saturated sodium chloride solution and driedover anhydrous MgSO₄. Evaporation of the solvent gives a white solidwhich is recrystallised from methanol and diethyl ether to give whitecrystals, identified by ¹ H nmr as 3-hexanoylindole (7a) (1.72 g, 8mmol, 80%), m.p. 153-154° C.

[Found; C,78.4; H, 8.1; N,6.5: Calc. for C₁₄ H₁₇ NO; C, 78.14; H, 7.9;N,6.5]

¹ H nmr (in CDCl₃ /DMSO d₆): 0.85 (t,3H), 1.32(m,4H), 1.7 (m,2H), 2.8(t,2H), 7.18(m,2H); 7.38 (dm, 1H), 7.82 (d, 1H), 8.3 (dm, 1H), 10.93(bs, N-H)

Preparation of 3-hexylindole (8a)

Red-Al [NaA6H₂ (OCH₂ CH₂ OCH₃)₂ ] (11.3 g, 3.4M in toluene), isdissolved in benzene (50 cm³), and suspended under nitrogen.3-hexanoylindole (7a) (5.59 g, 26 mmol) in benzene (30 cm³) is added ina drop-wise manner at 0° C. (zero)(ice cooled), and extra benzene (20cm³) is added later on. The reaction mixture is stirred at roomtemperature for three hours and at 50° C. for one hour. The reactionmixture is then cooled to room temperature and cautiously hydrolysedwith 100 cm³ of water. The white precipitate is removed by filtration.The organic layer is extracted using diethyl ether (5×25 cm³), and driedover MgSO₄. Evaporation of the solvent yields thick green liquid whichwas purified by dry column flash chromatography. The thick yellow liquidis distilled fractionally under vacuum and the pure product distilled at146° C., 1.5 mmHg. This was shown by ¹ H nmr to be pure 3-hexylindole(8a) (3.15 g, 15.7 mmol, 70%).

[Found; C, 83.3; H, 9.8; N, 7.3: Calc. for C₁₄ H₁₉ N; C,83.5; H, 9.5; N,7.0]

¹ H nmr (in CDCl₃): 1.02 (t, 3H), 1.4 (m, 6H), 1.8 (quintet, 2H), 2.85(t,2H), 6.92 (s, 1H), 7.18-7.38 (m, 3H), 7.65 (bs, N-H), 7.7 (d, 1H).

Preparation of 3-dodecanoyl-1-tosylindole (6b)

A solution of dodecanoyl chloride (5b) (5.37 g, 24.6 mmol) indichloromethane (20 cm³) is added slowly to a suspension of aluminiumchloride (3.0 g, 22.6 mmol) in dichloromethane (40 cm³) under N₂. Afterthe addition is complete the mixture is stirred for ten minutes, then1-tosylindole (4) (6.12 g, 22.6 mmol) in dichloromethane (20 cm³) isadded at 5° C. After two hours stirring at room temperature, thereaction mixture is treated with ice water. The organic layer isextracted with dichloromethane and dried over anyhdrous MgSO₄.Evaporation of the solvent gives a thick orange liquid whichcrystallises to a white solid when treated with a mixture of methanoland dichloromethane. This is filtered off and dried in vacuo, and isshown by ¹ H nmr to be a pure product of 3-dodecanoyl-1-tosylindole (6b)(9.0 g, 19.8 mmol, 87.7%), m.p. 79-80° C.

Found: C, 71.2; H, 7.5; N, 2.9; S, 7.4: Calc. for C₂₇ H₃₅ NSO₃ ; C,71.5,H, 7.7; N, 3.1; S, 7.1;)

¹ H nmr (in CDCl₃): 0.88 (t, 3H), 1.3 (m, 16H), 1.77 (quintet, 2H), 2.35(s, 3H), 2.89 (t, 2H), 7.28 (d, 2H, Ar), 7.35 (m,2H), 7.83 (d,2H), 7.93(dm, 1H), 8.22 (s, 1H), 8.35 (dm, 1H)

Preparation of 3-dodecanoylindole (7b)

3-dodecanoyl-1-tosylindole (6b) (6.7 g, 15 mmol) is dissolved in dioxane(90 cm³). Sodium hydroxide (5M, 90 cm³) is added to the solution, whichis stirred under reflux for 24 hours. The organic phase is separatedfrom the aqueous layer by extraction with diethyl ether. The combinedextracts are washed with saturated sodium chloride solution, and driedover anhydrous MgSO₄. Evaporation of the solvent yields a white solidwhich is recrystallised from methanol and diethyl ether to give whitecrystals, identified by ¹ H nmr as 3-dodecanoylindole (7b) (3.94 g, 13.2mmol, 88%), m.p. 129-130° C.

[Found: C, 80.4; H, 9.9: N, 4.7: Calc for C₂₀ H₂₉ NO; C,80.3: H,9.6; N,4.7.]

¹ H nmr (in CDCl₃ /DMSO d₆): 0.46 (t,3H); 0.9 (m,10H); 1.35 (quintet,2H); 2.42 (t,2H); 6.81(m,2H); 7.08(d,m,1H); 7.5(d, 2H); 7.9(dm, 2H);10.93 (bs, N-H).

Preparation of 3-dodecylindole (8b)

Red-A1[NaA1H2 (OCH2CH2OCH3)₂ ] (6.78 g, 3.4M in toluene) is dissolved indry benzene (40 cm³), and is suspended under nitrogen.3-dodecanoylindole (7b) (4.2 g, 14 mmol) in benzene (30cm³) is added ina drop-wise manner at 0° C. (ice cooled), and extra benzene (20cm³) isadded later on. The reaction mixture is stirred at room temperature for3 hours, and at 50° C. for 1 hour. The reaction mixture is then cooledto room temperature and cautiously hydrolysed using 100 cm³ water. Thewhite precipitate is removed by filtration. The organic layer isextracted using diethyl ether (5×25 cm³), and dried over MgSO₄.Evaporation of the solvent yields a thick green liquid which is purifiedby dry column flash chromatography (using a solvent mixture of 50%dichloromethane in petroleum ether b.p. 30-40° C.). Evaporation of thesolvents gives a white solid, which is shown by ¹ H nmr to be pure3-dodecylindole (8b) (2.65 g, 9.19 mmol, 65%)

[Found: C,84.1, H,11.2; N,5.1: Calc. for C₂₀ H₃₁ N; C,84.2; H, 10.9;N,4.9]

¹ H nmr (in CDCl₃): 0.85 (t,3H); 1.25(m, 10H); 1.7(quintet, 2H); 2,7(t,2H); 6.96 (s,1H); 7.1-7.2 (m,2HAr); 7.35 (d,1H); 7.6 (d,1H); 7.9(bs,N-H)

Gas sensors were fabricated using poly 3-hexanoylindole, poly3-dodecanoylindole, poly N-benzylindole and poly N-octylindole as theactive gas sensing elements. The sensors were exposed to pulses ofsaturated vapour of the compounds listed in Table 1 at room temperature.The responses of the polymers were detected by monitoring the change inDC resistance of the sensor on exposure to vapour.

                  TABLE 1                                                         ______________________________________                                        Compounds detected by the gas sensors.                                                     Saturated Vapour Pressure/mmHg                                                                   Concentration/ppm                               Compound                       (at 25° C.)               (at                                         25° C.)                                ______________________________________                                        acetic acid                                                                            14.4                            20235                                  propanoic acid                 3.9                            4902                                       butyric acid                   0.78                                                           85.5                               valeric acid                   0.21                           36.48                                      isovaleric acid                0.35                                                           0.998                              phenol                         0.45                           461.7                                      p-cresol                       0.22                                                           114                                4-ethylphenol                  0.06                           74.1          ______________________________________                                    

The responses of the various polymers are shown in FIGS. 5-12 andindicate that the polymers are sensitive to a range of compounds,including a number of phenols.

We claim:
 1. A semiconducting organic polymer polymerised from a3-substituted or 1,3-substituted indole monomer.
 2. A semiconductingorganic polymer polymerised from a 1-substituted, 3-substituted or1,3-substituted indole monomer for use in a gas sensor.
 3. Asemiconducting organic polymer according to claim 1 or claim 2 whereinthe substituent at the 3 position is an alkyl group.
 4. A semiconductingorganic polymer according to claim 1 or claim 2 wherein the substituentat the 3 position is an acetyl or an aromatic group.
 5. A semiconductingorganic polymer according to claim 1 or claim 2 in which the substituentat the 1 position is an alkyl group.
 6. A semiconducting organic polymeraccording to claim 1 or claim 2 comprising an alkyl or acetylsubstituent or substituents which substituent or substituents possessesat least six carbon atoms.
 7. A semiconducting organic polymer accordingto claim 1 or claim 2 in which the substituent at the 1 positioncontains an aromatic group.
 8. A semiconducting organic polymeraccording to claim 1 or claim 2 in which the substituent at the 1position is benzyl.
 9. A semiconducting organic polymer according toclaim 1 or claim 2 in which the substituent at the 1 position is tosyl.10. A semiconducting organic polymer according to claim 1 or claim 2polymerised electrochemically from a solution containing the monomer anda counter-ion.
 11. A semiconducting organic polymer according to claim 1or claim 2 polymerised electrochemically from a solution containing themonomer and a counter-ion in which the counter-ion is selected from thegroup comprising BF₄ ⁻, PF₆ ⁻,ClO₄ ⁻, C₈ H₁₇ SO₃ ⁻,[Fe(CN)₆ ]³⁻ or CH₃C₆ H₄ SO₃ ⁻.
 12. A semiconducting organic polymer according claim 1 orclaim 2 in which the monomer is selected from the groupcomprising:1-octylindole; 1-benzylindole; 1-tosylindole;3-hexanoxyl-1-tosylindole; 3-hexanoylindole; 3-hexylindole;3-dodecanoyl-1-tosylindole: 3-dodecanoylindole and 3-dodecylindole. 13.A gas sensor comprising:a pair of electrodes; one or more semiconductingorganic polymers of which at least one is a semiconducting organicpolymer according to claim 2 deposited between the pair of electrodes insuch manner as to effect a semiconducting electrical connection betweensaid electrodes; means for applying electric signal across saidelectrodes; and detection means for detecting a chosen electricalproperty in the presence of a gas.